Tesla told his story to the author of "Prodigal Genius" (1944), who wrote: "What Tesla had discovered was a means of creating a rotating magnetic field, a magnetic whirlwind in space which possessed fantastically new and intriguing properties...It was not necessary for him to construct models of copper and iron: in his mental workshop he constructed them in wide variety...The mental constructs were built with meticulous care as concerned size, strength, design and material; and they were tested mentally, he maintained, by having them run for weeks-after which time he would examine them thoroughly for signs of wear."